In a power supply device generally known, an output is obtained at a secondary side by resonating the voltage of a primary side coil of a switching transformer and a capacitor connected at its both ends.
As means for obtaining a stabilized output at the secondary side, a circuit composition for controlling the primary side, or a circuit composition for controlling the secondary side are used, among others.
First, as the means for controlling the primary side, FIG. 15 shows a circuit diagram of a conventional power supply device for controlling and stabilizing the on/off period of switching by installing a control circuit at the primary side, and feeding back the gate signal of the switching element as means for stabilizing and oscillating, to the output of the switching element by an impedance circuit composed of a series circuit of resistance and diode. According to the diagram, an input power source 1 is a DC voltage rectified and smoothed from a commercial power source, and a series circuit of starting circuit composed of a resistance 2 and a capacitor 3 is connected to both ends of the input power source 1, and a series circuit of a primary side coil 4 of switching transformer and switching element 5 is connected, and a capacitor 6 is connected to both ends of the primary side coil 4 of the switching transformer.
Moreover, the junction of the resistance 2 and capacitor 3 is connected to the drain of the switching element 5 through a series circuit of a resistance 7 and diode 8, and is further connected to the gate of the switching element 5 through a control winding 9 of the switching transformer. A capacitor 11 is connected to both ends of a secondary side coil 10 of the switching transformer, and a capacitor 13 is connected through a diode 12, thereby obtaining an output at both ends of the capacitor 13. Incidentally, the load side after the secondary side coil 10 of the switching transformer is separable, and an output can be obtained as required.
The operation of the conventional power supply device is described below. First, when the input power source 1 is applied, a voltage starts to be charged into the capacitor 3 through the resistance 2. The voltage of the capacitor 3 is fed into the gate of the switching element 5 through the control winding 9 of the switching transformer, and when reaching the threshold voltage of the gate, the switching element 5 begins to conduct. As a result, a voltage is induced in the control winding 9 of the switching transformer and the secondary side coil 10 of the switching transformer, and the voltage of the control winding 9 of the switching transformer elevates, and the gate voltage of the switching element 5 is further increased, so that the switching element 5 is completely turned on instantly by the positive feedback action.
Therefore, the current of the primary side coil 4 of the switching transformer, that is, the drain current of the switching element 5 increases linearly, and the energy is accumulated in the primary side coil 4 of the switching transformer. As the switching element 5 is completely turned on, an impedance circuit 14 of resistance 7 and diode 8 (or, an impedance circuit 15 shown in FIG. 15, instead of this impedance circuit 14) begins to discharge the voltage of the capacitor 3, that is, the gate voltage of the switching element 5. By such feedback action, when the gate voltage of the switching element 5 becomes lower than the threshold voltage, the switching element 5 is suddenly turned off.
As the switching element 5 is turned off, the voltage induced in the primary side coil 4 of the switching transformer is inverted, and resonance with the capacitor 6 occurs at the same time. When this resonance voltage is inverted again, it is driven to turn on the switching element 5 again through the control winding 9 of the switching transformer. At the same time, at the secondary side, too, resonance of the secondary side coil 10 of the switching transformer and the capacitor 11 occurs, and a DC output is supplied into the secondary side load 16 by a rectifying and smoothing circuit of diode 12 and capacitor 13.
Next, a prior art of controlling the secondary side output is described in a circuit diagram in FIG. 16. According to the diagram, reference numeral 20 denotes a primary side power supply unit, being composed of a DC input power source 21, a high frequency current generating circuit 22 connected thereto, a primary side resonance capacitor 23, and a primary side coil 24, and reference numeral 25 is a secondary side power source, which is provided in a separate housing from the primary side power supply unit 20, being composed of a secondary side coil 26, a secondary side resonance capacitor 27 connected to both ends of the secondary side coil 26, a secondary side rectifier 28, and an output capacitor 29 having one end connected to the secondary side rectifier 28, and other end connected to the secondary side coil 26, and moreover an output stabilizing circuit 30 and output capacitor 29 are connected, and a secondary side load (not shown) is connected to this output stabilizing circuit.
As described herein, since the prior art is intended to control either the primary side or the secondary side, it is used not only as a power supply device of a general electronic appliance, but also as a non-contact type power supply device having the primary side and secondary side provided in different housings.
However, in the circuit composition in FIG. 15, among the above conventional constitutions, the diode 8 used for feedback is required to have a high withstand voltage because a voltage in reverse direction is applied due to resonance of the primary side coil 4 of the switching transformer when the switching element 5 is turned off. Moreover, since the impedance of the control circuit is very high, a significant effect may be applied to the switching action of turn-on and turn-off of the switching element 5 due to reverse leak current of the diode 8, and the diode 8 is required to have a very small reverse leak current. Yet, for operation at high frequency of several hundreds of kHz, high frequency switching is required at the same time. The diode satisfying such characteristic is very hard to manufacture and very high in cost.
In the circuit composition in FIG. 16, incidentally, in order to obtain an output of high precision, a larger power loss occurs in the output stabilizing circuit 30.
It is hence an object of the invention to solve such problems and present a power supply device capable of obtaining a stable secondary output efficiently.